In common machine tools and the like, when machining a workpiece by a tool, the object is machined with a plurality of axes driven by electric motors cooperatively synchronizing with each other.
As a machine tool which executes machining with a plurality of axes cooperatively synchronizing with each other, for example, such a vertical-type machining center as illustrated in FIG. 1 is known. In the vertical-type machining center 100, a table 102 which fixes a workpiece 101 moves in an X-axis direction and a Y-axis direction, and a rotary tool 103 moves in a Z-axis direction, so that machining is executed.
In addition, as a machine tool which executes machining with a plurality of axes cooperatively synchronizing with each other, such a lathe as illustrated in FIG. 2 is known. In the lathe 200, a workpiece 201 is fixed on a rotary C-axis, and machining is executed by operating a tool 202 so as to be in contact with the workpiece 201 in a radial direction (X-axis) and a direction along the rotational axis (Z-axis).
Furthermore, as a machine tool which executes machining with a plurality of axes cooperatively synchronizing with each other, such a gear generating machining device as illustrated in FIG. 3 is known. In the gear generating machining device 300, a workpiece 301 is fixed on the rotary C-axis, and a tool 302 is rotated on a B-axis and an electric motor 304 for the B-axis and an electric motor 303 for the C-axis are controlled such that rotation of the B-axis and rotation of the C-axis synchronize with each other at a predetermined ratio (=the number of threads/the number of teeth), whereby machining is executed.
There is a case where the workpiece 101 is tapered on an X-Y plane of the vertical-type machining center 100. In this case, the electric motors for driving the X-axis and the Y-axis, respectively, operate synchronously at a speed corresponding to a taper angle θ (Vx=V cosθ, Vy=V sinθ). The relationship between the X-axis position and the Y-axis position of the tool 103 is illustrated in FIG. 4A.
Also when the cylindrical workpiece 201 is machined by the lathe 200, synchronous operation on the Z-axis is executed so as to be in proportional to a rotational speed around the C-axis. Relationship between a C-axis angle of the workpiece 201 and the Z-axis position of the tool 202 in this case is illustrated in FIG. 4B.
When the workpiece 301 is machined by the gear generating machining device 300, synchronous operation on the C-axis is executed so as to be in proportional to a rotational speed around the B-axis (ratio=the number of threads/the number of teeth). The relationship between, the C-axis angle of the workpiece 301 and the B-axis angle of the tool 302 in this case is illustrated in FIG. 4C.
When machining a workpiece with two axes synchronizing with each other as described in above cases, vibration may occur depending on machining load disturbance and rigidities of a tool, the workpiece and a mechanism unit which drives them. Further, the vibration might be amplified due to interference between axes caused by contact between the workpiece and the tool, thereby adversely affecting machining precision.
In such a case, a conventional practice reduces vibration by a method of suppressing vibration by lowering responsiveness of an electric motor which drives each axis or a method of independently controlling vibration damping on each axis.
For example, a method of reducing vibration for respective axes by a vibration reduction filter corresponding to machine rigidity is known (e.g. Japanese Patent Publication No. 4658181 (JP4658181B). Although this conventional art enables reduction in vibration on a single axis, it has a problem that an effect of reducing vibration caused by interference between axes is insufficient.
On the other hand, as a method of reducing vibration due to interference between axes, a vibration damping control method for correcting a torque command by using a speed difference between two electric motors when they drive one movable member (e.g. Japanese Patent Publication No. 3492583 (JP3492583B)) is known. According to this conventional art, two electric motors are fixedly coupled and the two electric motors are driven at the same speed. Accordingly, the conventional art does not relate to a method of reducing vibration caused when an object is machined with a tool in contact therewith and cannot therefore reduce vibration caused when axes synchronize with each other at a different speed. In addition, the conventional art has a problem that the art cannot treat a non-coupling state such as a relationship between a tool and a workpiece.
An object of the present invention is to provide a servo controller which reduces vibration due to interference between axes caused when machining is executed while a plurality of axes driven by electric motors synchronize with each other.